docs: codegen is delegated to freenet-migrate-build

[sanity]

Problem

Two agent-facing rule files describe the legacy-delegate codegen as something River's own build.rs performs. That stopped being true when River adopted freenet-migrate-build (#398): the build script is now only the entry point, and the generation is done by the crate.

Stale statements:

• .claude/rules/delegate-migration.md:34 "The UI's build.rs generates Rust code from it at compile time"
• .claude/rules/delegate-migration.md:90 key-files row "ui/build.rs | Generates LEGACY_DELEGATES const from the delegate TOML"
• .claude/rules/river-publish.md:163-164 "LEGACY_DELEGATES is generated at compile time from legacy_delegates.toml by ui/build.rs."

common/build.rs has the same shape for the room-contract registry and was described the same way at delegate-migration.md:44.

Approach

Correct the attribution without inflating it. ui/build.rs is still the entry point, so the fix names the delegation rather than rewriting the surrounding procedure:

• ui/build.rs:125 calls freenet_migrate_build::codegen() with .entry_registry("../legacy_delegates.toml", Component::Delegate) and .delegate_pair_view("LEGACY_DELEGATES").
• common/build.rs:29 calls the same freenet_migrate_build::codegen() with Component::Contract and .contract_hash_view("LEGACY_ROOM_CONTRACT_CODE_HASHES").
• What each build script still owns locally is the append-only emptiness gate that asserts on the generated output (ui/build.rs:143-158, common/build.rs:41-46). The wording says so, since that is the part a reader would otherwise assume moved into the crate too.

freenet-migrate-build is pinned at 0.2 in ui/Cargo.toml:108 and common/Cargo.toml:77. Note this is a separate crate from the runtime half freenet-migrate, which is at 0.5 (ui/Cargo.toml:98); the two are at different versions and the doc text does not cite a version for either.

river-publish.md already carries an accurate crate-walk section at :239, so that content is not duplicated. The only addition there is a pointer to the new freenet-migrate-adoption skill from the existing "Release 2 retires the sweep" line.

Testing

Documentation only, no code paths touched. Every claim was read from origin/main source rather than the working tree.

[AI-assisted - Claude]